Income Taxes - Narrative (Details)	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Aug. 31, 2016 change
Operating Loss Carryforwards [Line Items]
Tax reform, revaluation of deferred tax assets		$ 9,400,000
Tax reform, valuation allowance reduction		9,400,000
Cash paid for income taxes	$ 0	16,750,000
Valuation allowance	55,348,000	30,850,000
Increase in deferred tax asset valuation allowance	24,500,000
Number of previous ownership changes | change			3
Unrecognized tax benefits	0	$ 0
Interest and penalty charges incurred	0
Federal Tax Authority
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	136,400,000
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	138,700,000
Research and Development Tax Credit Carryforwards | Federal Tax Authority
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	9,300,000
Research and Development Tax Credit Carryforwards | State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	3,800,000
Investment Tax Credit Carryforwards | State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	$ 600,000